The N-terminal lysine residues of histones H3 and H4 are generally underacetylated in chromatin of transcriptionally inactive chromosomal regions. The acetylated state of a chromosomal domain may act to mark and maintain active or inactive regions in different cellular lineages and throughout the cell cycle. A novel role for histone acetylation in the regulation of centromeres has been identified. Exposure of yeast cells to Trichostatin A (TSA), a specific inhibitor of histone deacetylase, interferes with repression of marker genes in centromeric heterochromatin, leading to chromosome loss, and disruption of the localization of Swi6p, a component of centromeric heterochromatin. Thus, the assembly of a fully functional centromere is partly imprinted in the underacetylated or transcriptionally silent state of centromeric chromatin (Cell 91:1021). We are investigating the involvement of histone deacetylase and proteins that may anchor a histone deacetylase to heterochromatic regions as part of a chromatin bound complex. Preliminary results using an antibody to Swi6p in conjunction with serial section immuno-EM and 3-D reconstruction have localized heterochromatin in wild-type cells. We are documenting the structure of regions of the nucleus containing heterochromatin, using HVEM electron tomography. Finally, we will investigate the changes in patterns of acetylation in rik1 mutant cells, which fail to localize Swi6p properly and are defective in centromere silencing. C5